Technical Field
[0001] The present invention relates to the use of a method for drying of a material, whereby:
a) the temperature of the material is adjusted to a value which is not injurious to
the material;
b) the material is fed into a vacuum chamber,
c) the material is led through the vacuum chamber without any heat supply to the material,
and
d) the dried material is removed from the vacuum chamber through an air lock.
Background Art
[0002] It is often difficult to remove the remaining water when preparing sugars or sugar
alcohols which are sensitive as well as hygroscopic products in dry form, such as
in powder form.
[0003] One of the reasons is the high content of hydrophilic groups in the above materials,
rendering the products hygroscopic.
[0004] Another reason is the tendency of some of said materials to form supersaturated solutions,
wherefrom it is difficult to precipitate and isolate solid products.
[0005] Supersaturated and other concentrated solutions are very often highly viscous. Consequently
they are difficult to handle and have the tendency to stick to the apparatus.
[0006] All these properties further impede the removal of water, e.g. by evaporation or
drying, since problems arise during the heating of the material. An equal distribution
of heat is, for example, not ensured, thus risking local overheating. During the heating
the material can be destroyed or deteriorate e.g. by burning, caramelization, denaturation
or another form of decomposition.
[0007] Several methods for drying products in order to remove the remaining water are known,
such as spray drying, drum drying, freeze drying or flash drying.
[0008] For spray drying, the solution to be dried is fed into a chamber in form of tiny
drops. The falling drops are dried by means of hot air so that the drops are transformed
into a dry powder before they reach the bottom of the chamber. Spray drying cannot
be used if the solution has the tendency to remain liquid, either as an supersaturated
solution or in form of a melt, during the drying process, where the temperature is
usually above 60°C, since the material accumulates on the walls of the spray drying
chamber.
[0009] Drum drying is normally performed at temperatures about 100°C. At this temperature
many materials occur in form of a melt and thus they cannot be transformed into a
solid product. During drum drying the product accumulates on the warm surfaces, causing
overheating with subsequent destruction or deterioration of the material.
[0010] With conventional flash drying water is removed almost instantaneously from wet,
solid particles, said particles being dispersed at high speed in a warm stream of
gas. In flash drying the temperature of the drying air is above 100°C, rendering this
drying method unsuitable for drying heat-sensitive products.
[0011] It is evident that neither spray drying, drum drying nor flash drying are suitable
for the preparation of solid, dry products, such as some sugars and some sugar alcohols,
other heat-sensitive products and/or those difficult to crystallize.
[0012] The only known methods suitable for drying such materials are freeze drying and microwave
vacuum drying. These methods are, however, expensive, since their operational costs
are high, especially with regard to energy consumption and capital costs.
[0013] The German Offenlegungsschrift No. 34 07 374 discloses a method for preparing dried
products from sucrose syrup. According to this method the pre-concentrated sucrose
syrup with a dry matter content of at least 70% is heated for a short period, such
as below 60 sec, to a very high temperature, and the warm material is expanded to
a concentrated syrup with a dry matter content of at least 90%. This syrup is transformed
into a dry, pourable product by sudden cooling and subsequent release of the remaining
water during crystallization.
[0014] This method is limited to easily crystallizable materials with a positive enthalpy
of crystallization, i.e. materials crystallizing during cooling. The method is consequently
unsuitable for the preparation of amorphous products and other materials difficult
to crystallize. Furthermore the material is subjected to high temperatures of about
135-155°C, thus rendering the method unsuitable for heat-sensitive materials.
[0015] US-PS No. 1.250.496 discloses a process for drying grain and other coarse, granular
materials, where the grain is heated and then subjected to first low vacuum and thereafter
high vacuum. The process involves expensive equipment and the process cannot be used
for drying particle-free, syrups.
[0016] US-PS No. 3.206.866 discloses a method and apparatus for dehydrating food employing
geothermal steam. The food is fed into a vacuum chamber through an air lock and is
transported through said chamber by means of several conveyor belts and is finally
removed from the vacuum chamber through an air lock. The food is heated inside the
vacuum chamber. The latter step renders this method unsuitable for dehydration of
syrups since the material is locally overheated resulting in its deterioration.
[0017] US-PS No. 4.574.495 discloses a drying apparatus with a vacuum chamber, wherein the
material to be dried is transported by means of conveyor belts. This apparatus is
also equipped with means for supplying heat to the inside of the vacuum chamber. Therefore
this apparatus also deteriorates a syrup due to overheating.
[0018] Finally EP Patent Application No. 0.231.584 A1 discloses a drying apparatus including
a screw conveyor. The apparatus is heated by means of a heating mantle. Therefore
also this apparatus is unsuitable for drying heat-sensitive materials, such as syrup-like
material.
[0019] Other patents, such as SE-PS No. 342.896, SE-PS No. 374.811, US-PS 3.698.098 and
US-PS No. 1.161.603, disclose various methods and apparatuses for drying materials,
such as wood panels, protinaceous materials, heat-sensitive parachutes of synthetic
fibres and colloid substances, respectively.
[0020] GB-PS No. 1.498.119 discloses a process for drying and expanding a paste, which is
extruded into a vacuum chamber. The temperature of the paste is between 60 and 125°C.
The known process is difficult to control, if the extruder aperture has a diameter
of less than 0.25 mm. In order to effect an adequate control of the process the aperture
should be of a diameter of from 0.5 to 5 mm.
[0021] DE-OS No. 2.744.099 discloses a method for preparing dextrose powder in dry form
containing a major amount of β-dextrose anhydride. This method, however, involves
a grafted crystallization and is not usable in case of non-crystallizing materials,
such as a mixture of oligosaccharides prepared according to the concurrent DK patent
application No. 1592/88, filed March 23, 1988.
[0022] P.E. Andersen and J. Risum (Introduktion til Levnedsmiddel-Teknologien, vol. 1, 3.
edition, p. 333, 1982, Polyteknisk Forlag, Copenhagen) disclose a conventional flash
evaporator. The evaporator is, however, only suitable for the preparation of concentrated,
still liquid products.
Disclosure of the Invention
[0023] The object of the present invention is to find a useful method for removing the remaining
water from heat-sensitive, hard or impossible to crystallize and/or hygroscopic syrups
by drying such syrup with a high dry matter content. Said method avoids the above
difficulties of the known methods and is less expensive than freeze drying and microwave
vacuum drying.
[0024] Accordingly, the invention relates to the use of a method for continuous drying of
a material, whereby
a) the temperature of the material is adjusted to a value below the boiling point
of said material at atmospheric pressure,
b) the material is fed into a vacuum chamber,
c) the material is led through the vacuum chamber without any heat supply to the material,
and
d) the dried material is removed from the vacuum chamber through an air lock,
which use is characterised in that the material is a substantially particle-free
syrup selected from the group consisting of syrups of carbohydrate and syrups of sugar
alcohol, and that the temperature of said syrup in step a) is adjusted to a value
below the boiling point of said material at atmospheric pressure but sufficiently
high for the material to be dried in step c) to an amorphous solid product.
[0025] The previously mentioned methods known from GB patent No. 1,498,119 and US patent
No. 1,250,496, respectively, share substantial points of resemblance with the method
used according to the invention. However, the known methods are used for the drying
of another type of material, such as extracts of coffee, chicory and other coffee
substitutes, tea and herbal extracts which are extruded as powder or paste in the
former and grain in the latter of the known methods. It is thus a question of another
type of starting material than those dried by the method used according to the invention
where such are used as starting material which may especially be defined as particle-free
syrups of carbohydrate and/or sugar alcohol. All the examples of the above GB patent
relate to products which have already been freeze dried or spray dried. By the known
drying processes it is thus not a question of a phase shift from a liquid to a solid
product. According to the above GB patent, the drying product is disintegrated before
it has finished expanding. By the method used according to the invention, the expansion
takes place momentarily after the dosing. The product is not subjected to the optional
coarse grinding until after the drying.
[0026] Although it is known to carry out a continuous drying of a material where
a) the temperature of the material is adjusted to a value which is not injurious to
the material,
b) the material is fed into a vacuum chamber,
c) the material is led through the vacuum chamber without heat being supplied to the
material, and
c) the dried material is removed from the vacuum chamber through an air lock,
it is new to use such method for continuous drying of a substantially particle-free
syrup of carbonhydrate and/or sugar alcohol, where the temperature of the syrup in
step a) is adjusted to a value below the boiling point of the material at atmospheric
pressure and where an amorphous, solid product is formed by the drying.
[0027] The person skilled in the art has not previously thought about using a method having
the above known features for drying of syrups per se, that is such materials which
are hygroscopic and which tend to form oversaturated solutions which are highly viscous
and therefore difficult to handle and which tend to adhere to the apparatus used.
Materials which are difficult to crystallize pose special problems.
[0028] The present invention was originally the result of the efforts to solve the task
of drying the mixture of inulides, which is described in Applicant's Danish patent
application No. 1592/88, as this mixture proved especially difficult to work with,
as a solid product cannot be formed by crystallization, as is the case with a syrup
of saccharose.
[0029] To solve such task, the person skilled in the art would traditionally look for a
corresponding known method and the most obvious method would be the method used for
drying fruit syrups. When such fruit syrups are to be dried, maltodextrin is conventionsally
added, as this permits spray drying of the syrup. This solution is, however, not particularly
attractive as products are obtained having a content of up to 50% maltodextrin, which
must be considered an unwanted filler. Thus, if this known method for drying of the
above inulin mixture was used, the product obtained would be less sweet and it would
possess less body.
[0030] As according to prior art, drying of the type of syrups dealt with here has not previously
been carried out, the person skilled in the art could not know whether the method
was practicable without encountering severe problems with deposits on the walls of
the vacuum chamber which would make it impossible to solve the task in practice. However,
it has been found that the method known from drying of for instance pasta-like products
may advantageously be used for these particularly difficult syrups and, surprisingly,
it has been found that as a result, a product is obtained which has particularly excellent
properties, as the product is an amorphous solid product.
[0031] Although the inventive use has been developed to dry the above particularly difficult
mixture of inulides, it has also proved suitable for drying crystallizable types of
sugar, e.g. saccharose. By using the method for such starting materials, the end product
prepared would deviate from the product obtained by the conventional crystallization
method. Thus, the product prepared by the inventive use has a special amorphous structure
which makes the product suitable for forming agglomerates. Such agglomerates are advantageous.
They are very easy to mix with other, they have an improved flowability, are less
dusty and display a lesser tendency to absorb moisture from the atmosphere. Furthermore.
it is possible to obtain agglomerated products with a high bulk density.
[0032] The resulting special structure has very special functional properties which, in
addition to the above, also include an aroma-carrying effect. The desirable properties
of the amorphous structures of types of sugar are described in an article by E.A.
Niediek: "Effect of Processing on the Physical State and Aroma Sorption Properties
of Carbohydrates". In said article, Niediek reaches the conclusion: "Economical methods
for the precise production of amorphous substances do not exist yet. However, once
they are developed, amorphous substances will find a large market because of their
many desirable functional properties". It appears from this conclusion that the present
invention will supply a large demand on the market.
[0033] The product obtained by the method has a particular amorphous structure rendering
it suitable to form agglomerates. Such agglomerates are advantageous because they
are very easily admixed other materials. Compared to crystalline products they have
a better flowability due to their smaller surface, a smaller amount of dust and display
a lesser tendency to absorb humidity from the atmosphere. Furthermore it is possible
to obtain agglomerate products having a high bulk density.
[0034] Furthermore solid and non-tacky materials having a dry matter content of only 95%
by weight can be prepared by the method.
[0035] The starting material of the method is a material concentrated by means of conventional
methods, e.g. evaporation. The degree of concentration depends on the material, since
the danger of destruction or deterioration, energy consumption and rheologic properties
of the concentrate have to be taken into consideration.
[0036] The method normally removes 2-9% by weight of water based on the feed. This is, for
example, used for drying an oligosaccharide with a dry matter content of 91-95% by
weight into a powder with a dry matter content of 95-99% by weight.
[0037] The method is suitable for drying a mixture of oligosaccharides with a general formula
GF
n, wherein G is glucose, F is fructose and n is an integer, said mixture being further
described in the concurrent patent application DK Patent Application No. 1592/88 and
comprising 10-20% weight of G + F + GF, 10-20% by weight of GF₂, 8-15% by weight of
GF₃ and 72-45% by weight of GF₄ and above.
[0038] This mixture is obtained from plant tubers or roots, especially the tubers of the
Jerusalem artichoke, Helianthus tuberosus L. or roots of chicory, Cichorium, using
a conventional plant for treating sugar beets to prepare a syrup of a dry matter content
of 65-80% by weight. This syrup is further evaporated by means of a suitable evaporator,
such as a falling film evaporator, a vertical vacuum dryer and a thin film evaporator,
to a dry matter content of 91-95% by weight before it is subjected to the inventive
method. The material is removed from the evaporator at a temperature of 80-100°C.
It is necessary to maintain this temperature, since otherwise the material turns viscous
and thus accumulates on the walls of the evaporator, resulting in an interruption
of its operation and destruction of the product.
[0039] For the method the temperature of the material is adjusted to a value below its boiling
point. The enthalpy of the material is such that no heat supply is necessary during
the subsequent steps (b, c and d).
[0040] The material is then fed into a vacuum chamber, preferably by distributing it corresponding
to a thin layer, in dropform, or in any other way ensuring a large surface of the
material.
[0041] The vacuum chamber is connected to a vacuum pump or the like to establish a suitable
vacuum.
[0042] When the material has entered the vacumm chamber the boiling point of the material
is lower than the temperature of the material at the pressure in the vacuum chamber,
causing a spontaneous evaporation of water. The heat of evaporation of water is taken
from the material resulting in a corresponding drop in temperature. It is thus unnecessary
to supply external heat. Supplying external heat during evaporation would cause undesired
local overheating. In the present case external supply is avoided thus simplifying
the process and reducing the costs for the equipment. Furthermore, the material is
hot while it has the highest water content.
[0043] During the evaporation of water in a vacuum chamber the material is cooled down to
a temperature slightly above the temperature where the vapor pressure of water corresponds
to the absolute pressure in the vacuum chamber. The vapor pressure of water at 22,
25, 30, 35 and 38°C is 19.8, 23.8, 31.8, 42.2 and 49.7 mmHg respectively. When the
vacuum chamber has an absolute pressure of 23.8 mmHg the product leaves the vacuum
chamber due to boiling point elevation with a temperature of approx. 27-30°C, while
an absolute pressure of 42.2 mmHg results in a temperature of 37-40°C.
[0044] The dry and cold product is removed from the vacuum chamber through an air lock,
e.g. a cell air lock, ensuring a continuous running of the assembly.
[0045] This way of drying is just as gentle as freeze drying, but considerably less expensive.
In case of the above mixture on the basis of tubers of Jerusalem artichokes, described
in the concurrent Danish Patent Application No. 1592/88, the energy costs for concentrating
the material from a dry matter content of 66% by weight to 98-99% by weight are 0.80
DKK/kg when freeze-drying is used. In comparison the total energy costs for concentrating
the same material in three steps comprising the inventive method, i.e. from 66% by
weight to 85% by weight in a falling film evaporator, from 85% by weight to 92% by
weight by means of batch evaporation in a vertical vacuum dryer and from 92% by weight
to 98-99% by weight according to the inventive method are only 0.11 DKK/kg.
[0046] When drying a syrup of a dry matter content of 91-95% by weight the temperature of
the syrup is adjusted to 80-100°C, preferably 90-100°C. This is advantageously achieved
by maintaining the temperature of the syrup leaving the pre-evaporation step.
[0047] The absolute pressure in the vacuum chamber is kept at 10-60 mmHg, preferably 20-50
mmHg. The material is carried through the chamber by a means of transport or by free
fall and leaves the vacuum chamber through an air lock, optionally subsequent to grinding.
The powder obtained in this manner is of a dry matter content of 95-99% by weight
and its temperature has dropped to 25-40°C.
[0048] For setting a suitable duration time in the chamber the velocity of the means of
transport is preferably adjustable or controlable. The basis for such a control is
e.g. the dry matter content of the finished product, the temperature of the product
when leaving the air lock or other values.
[0049] For the method to be carried out in a suitable and reliable manner the following
data has to be in a matching, dynamic balance:
- composition of the starting material, incl. its water content,
- flow rate of the starting material,
- temperature of the starting material,
- pressure in the vacuum chamber,
- transport velocity through the chamber and
- temperature and dry matter content of the finished product when leaving the vacuum
chamber.
[0050] In order to ensure such a dynamic balance the method is preferably carried out in
the following way:
a) the temperature of the material is adjusted to a temperature less than 30°C below
the boiling point of the material, preferably less than 10°C below the boiling point,
b) the material is fed into a vacuum chamber having an absolut pressure of 10-60 mmHg,
c) the material is led through the vacuum chamber by a means of transport,
d) the dried material is removed from the vacuum chamber through an air lock, optionally
subsequent to a preceding gross grinding.
[0051] In order to obtain a suitable flow of the material through the vacuum chamber, the
velocity of the means of transport is advantageously adjustable.
[0052] In an alternative, preferred embodiment also ensuring the above dynamic balance
a) the temperature of the material is adjusted to a temperature less than 30°C below
the boiling point of the material, preferably less than 10°C below the boiling point,
b) the material is fed into a vacuum chamber having an absolut pressure of 10-60 mmHg,
c) the material is led through the vacuum chamber by means of free fall,
d) the dried material is removed from the vacuum chamber through an air lock, optionally
subsequent to a preceding gross grinding.
[0053] For obtaining a suitable dry matter content of the powder the material fed into the
vacuum chamber is advantageously of a dry matter content of 91-95% by weight.
[0054] The method is especially suitable for drying of materials, such as syrups comprising
carbohydrates; syrups comprising sugar alcohol; honey; fruit juices and vegetable
juices. Accordingly the method is potentially suitable for drying e.g. invert syrup,
isosyrup (high fructose syrup, HFCS), enriched high fructose syrup (EFCS) and glucose
syrup; sorbitol and xylitol; vegetable and fruit juices, such as carrot juice, tomato
juice or apple juice; and the above mixture of saccharides mentioned in the concurrent
DK Patent Application No. 1592/88.
[0055] Examples of the composition of the above HFCS and EFCS are:
HFCS: |
42% by weight of fructose |
5% by weight of higher sugars |
53% by weight of glucose |
EFCS: |
55% by weight of fructose |
5% by weight of higher sugars |
40% by weight of glucose |
[0056] For ensuring a substantially complete and fast evaporation of water the means of
transport is advantageously a conveyor belt and the material is distributed on the
conveyor belt in an amount corresponding to a layer with a thickness of 1-10 mm, preferably
2-5 mm. Such a layer is, however, never formed in practice, since the material foams
up immediately upon entering the vacuum chamber.
[0057] Excellent transport characteristics are alternatively achieved by employing a screw
conveyor. The screw conveyor is preferably a self-cleaning twin screw to prevent the
accumulation of material on the screw. A suitable fast transportation rate of the
material through the vacuum chamber is obtained by employing several screws, for example
2-6 and preferably 2-5, said screws being parallel and adjacent to each other.
[0058] When 2 or more screws are used they can rotate in the same or opposite direction,
as righthanded as well as lefthanded screws may be used.
[0059] According to the invention the use of the method can be carried out by means of an
assembly characterized in that it comprises a means for adjusting the temperature
of the material to a value below the boiling point of the material at atmospheric
pressure, a feeding means for feeding the material into a vacuum chamber, a vacuum
chamber, a means of transport for carrying the material through the vacuum chamber,
and an air lock.
[0060] In one embodiment of the assembly the means of transport is a conveyor belt made
of e.g. steel, plastic, rubber or other suitable material.
[0061] The feeding means is provided with a device for distributing the material by means
of extrusion on the conveyor as a layer, preferably corresponding to a thickness of
1-10 mm, more preferred 2-5 mm. As mentioned this layer is only formed theoretically,
in practice the material immediately foams up. During transport through the vacuum
chamber the material thus foams up, as the water evaporates fast, causing an immediate
drop in temperature. At the end of the conveyor belt a knife or scraper scrapes the
dried material off the conveyor belt, whereupon it falls into a screw conveyor.
[0062] The screw conveyor - also under vacuum - crushes the product and transports it to
an air lock, such as a cell air lock, wherefrom the dried product is continously removed
in form of a powder. The crushing also ensures that the product is able to pass through
the air lock.
[0063] In an alternative embodiment of the assembly the means of transport is formed like
a screw conveyor, e.g. a twin-screw conveyor. In this embodiment the material is fed
onto the screw through an extruder or by spraying it through a nozzle, and carried
towards the output side by the screw. The material is distributed corresponding to
a thin layer on the surface of the screw, said layer instantaneously foaming up by
the sudden evaporation of water. The resulting foam is crushed and ground by the screw
in such a way as to enable the dried product to fall into a hopper at the output side
to be removed through an air lock, preferably a cell air lock.
[0064] If the product is very sticky, as is the case of many syrups, the screw is advantageously
a self-cleaning screw. A self-cleaning twin screw includes two screws, one of them
rotating twice as fast as and having a pitch half a large as the other one. The self-cleaning
screw is provided with a self-cleaning rounded section so that the two screws clean
each other. Usually there is a gap of 3 mm between the two screws, for especially
fine processing, however, the self-cleaning twin screw can be manufactured with a
gap of down to about 1 mm.
[0065] In both embodiments the amount of material to be fed and the velocity or speed of
the belt or screw are determined in such a way that the layer theoretically formed
on the belt or screw is sufficiently thin.
[0066] It is thus advantageous to render the velocity or speed of the belt or screw adjustable
or controlable.
[0067] In a further embodiment the means for carrying the material through the vacuum chamber
is provided by free fall. This assembly is especially suitable for drying sticky materials
difficult to remove from the means of transport. This embodiment constitutes a simple
and inexpensive alternative to the self-cleaning screw.
[0068] An assembly according to this embodiment suitably comprises a means for adjusting
the temperature of the material to a value below the boiling point of the material
at atmospheric pressure; a feeding means for feeding the material through apertures
into the top of the vacuum chamber; a vacuum chamber provided for free fall; a hopper
at the bottom of the vacuum chamber for collecting the dried material; a beater situated
inside the hopper and an air lock.
[0069] In a simple form of an assembly according to this invention the air lock is a ball
valve.
[0070] In all above embodiments the feeding means is preferably provided with several apertures
or one or more narrow gaps or slots, the most narrow dimension of said apertures,
gaps or slots being not more than 2 mm, preferably not more than 1 mm, most preferably
not more than 0.25 mm. The narrow dimension ensures a good distribution of the material
throughout the vacuum chamber as well as a short drying period.
[0071] Further scope of applicability of the present invention will become apparent from
the detailed description given hereinafter. However, it should be understood, that
the detailed description and specific examples, while indicating preferred embodiments
of the invention, are given by way of illustration only, since various changes and
modifications within the spirit and scope of the invention will become apparent to
those skilled in the art from this detailed description.
Brief Description of the Drawings
[0072] The invention is explained below in greater detail and with reference to the accompanying
drawings, in which
Fig. 1 shows a first embodiment of the assembly, where the means of transport is a
conveyor belt, and
Fig. 2 shows another embodiment of the assembly where the means of transport is a
twin-screw conveyor.
Fig. 3 shows the relation between the dry matter content of the starting material
and the material leaving the assembly and the temperature by means of the method when
flash drying glucose syrup as described in Example 8.
Fig. 4 shows a third embodiment of the assembly using free fall as a means of transport.
Best Mode for Carrying out the Invention
[0073] Fig. 1 illustrates a vacuum chamber 2 inside a housing 4. To resist the external
pressure the housing 4 has the shape of a cylinder with convex ends. The vacuum chamber
2 is connected to a vacuum source (not shown) via a connection piece 6 and corresponding
channels (not shown). The vacuum pump is preferably a water ring vacuum pump. Between
the vacuum pump and the vacuum chamber a condenser can be installed, where the steam
from the vacuum chamber condensates. The installation of a condenser increases the
efficiency of the vacuum pump. The material to be dried is fed into a channel 8 connected
to a distributing piece 10. The distributing piece 10 can be provided with several
small apertures or one or more small gaps or slots, said apertures, gaps or slots
optionally being of varying length or width. The distributing piece distributes the
material in a uniform amount across the entire width of the distributing piece.
[0074] A conveyor belt 12 runs on two rolls 14 with adjustable speed. The distributing piece
10 distributes the material over one end of the conveyor belt 12. By choosing a suitable
adjustment of the feeding rate through the channel 8 and the distributing piece 10
and of the velocity of the conveyor belt 12 the material is distributed across the
width of the conveyor belt 12 in an amount corresponding to a uniform, thin layer.
During the transport through the vacuum chamber 2 water evaporates from the material
16 and the material foams up. When it has reached the opposite end of the conveyor
belt 12 the material has been converted into a dry and solid product and is scraped
off by a scraper 18.
[0075] The scraped-off product falls into a screw 20 in a chamber 22. The chamber 22 is
connected to the vacuum chamber 2 and is thus also exposed to vacuum.
[0076] The screw 20 transports the product to a cell air lock 24, simultaneously crushing
and grinding said product. The finished product 26 is removed through the air lock
24.
[0077] Fig. 2 shows an alternative embodiment of the assembly. A vacuum chamber 102 is inside
a housing 104 of cylindric shape with convex ends as in the embodiment of Fig. 1.
The vacuum chamber 102 is connected to a vacuum source via a connection piece 106
and channels (not shown). The material is fed through a channel 108 directly to the
input end of a speed-adjustable twin screw 112. The material feeding rate through
the channel 108 and the speed of the twin screw 112 is adjustable as to allow the
material to be distributed corresponding to a thin and uniform layer on the surfaces
of the twin screw 112. During transport through the vacuum chamber 102 water evaporates
from the material 116 and the material foams up. Simultaneously, the twin screw crushes
and grinds the material. As a result, the material reaches the output end of the twin
screw as a dry, pulverized product. The product falls directly into a cell air lock
124, optionally by employing an internal funnel. The finished product 126 is removed
through the air lock 124.
[0078] Fig. 4 shows an alternative embodiment of the assembly. A vacuum chamber 202 inside
a housing is connected to a vacuum pump 205 via a connection piece 206. The material
is fed to a distributing piece 210 via a channel 208. The distributing piece 210 has
one or more small apertures for instance having a diameter of 1 mm. The dried material
falls into a hopper 211, wherein a beater 213 disintegrates the dried material. The
disintegration allows the removal of the material through a ball valve 215. When the
ball valve is filled it is turned 180° and the finished product 226 is removed. The
ball valve 215 is provided with a suction conduit to the vacuum pump 205 ensuring
that no air leaks into the vacuum chamber when returning the ball valve 215. Between
the vacuum pump 205 and the vacuum chamber 202 a condenser 228 is installed, said
condenser being fed with cooling water 230. The steam from the vacuum chamber is condensated
in the condenser 228 thus increasing the efficiency of the vacuum pump.
[0079] The invention is described below in greater detail by means of the following examples.
Example 1 describes the preparation of the starting material, while Examples 2-10
describe the method used according to the invention.
Example 1: Preparation of a syrup.
[0080] The harvested tubers of the Jerusalem artichoke are treated on a conventional plant
for treating sugar beets. The treatment includes the following steps.
1. Feeding and removal of stones and grass
[0081] The tubers are emptied into a beet yard and flow into the plant, while stones as
well as green plant material (i.e. grass and stem material) are removed. Most of the
soil is also washed off.
2. Cutting
[0082] For preparing the tubers for to the subsequent extraction process said tubers are
cut into cosettes with a cross-section of approx. 0.5 x 0.5 cm. Their length depends
on the size of the tubers (typically 2-5 cm). The cutting process is performed on
a conventional sugar beet cutter. It can, however, be necessary to use other knives.
3. Extraction
[0083] In order to extract the desired product from the cosettes, the extraction process
is performed analogous to the one known from the extraction of sugar from sugar beets.
The extraction is performed in a so-called DDS-diffusor, a trough with a steam mantle.
The trough has a small inclination and is provided with a twin screw ensuring transport
of the cosettes.
[0084] The cosettes are extracted according to the counterflow principle, i.e. the cosettes
are fed through a funnel in the bottom part of the trough. Water as well as the press
juice obtained in step 4 are fed into the top part of the trough.
[0085] The cosettes are then transported counter to the flow of water, whereby oligosaccharides
and other water-soluble components, such as salts and proteins, pass into the water
phase.
[0086] The temperature during the extraction is between 60-85°C. Such a high temperature
ensures not only a good solubility of oligosaccharides but also partially denaturates
the protein as to render it insoluble. Enzymes are also denaturated and thus inactivated
at this temperature.
[0087] The dry matter content of the extract is 10-17% by weight.
4. Pressing of the pulp
[0088] The extracted cosettes are pressed in a special press of the type also used for conventional
sugar beet processing. This is done to increase both the yield of oligosaccharides
as well as the dry matter content of the pulp. The pulp has often to be dried with
regard to stability during transport and storage until use, e.g. in form of foodstuffs.
The increase in yield is achieved by transferring the press juice back to the extraction
process, as described above.
5. Purification of the juice
[0089] The juice obtained by the extraction process is turbid since it contains particulate
and colloidal material. Amongst the impurities present are pectin and proteins as
well as cell material from the cosettes.
[0090] In order to remove these impurities slaked lime, Ca(OH)₂ is added up to a pH-value
of 10.5-11.5, thereby precipitating a part of the impurities.
[0091] The pH-value is lowered again by adding CO₂ or phosphoric acid either before or after
filtration. Thus excess calcium is precipitated either as calcium carbonate or calcium
phosphate. The pH-value after this treatment is between 8.0 and 9.5. The juice is
subsequently filtered. The temperature during the lime treatment is 35-40°C, and during
the lowering of the pH-value and the filtering it is 60-80°C. Precipitation and filtering
are improved at the higher temperature.
[0092] The purification of the juice is performed using the same equipment as in conventional
sugar beet processing.
[0093] After the purification the dry matter content is 9-16% by weight.
6. Ion exchange
[0094] After the purification the juice still contains salts (3-8% by weight of the total
dry matter) and it is brownish or greenish in colour. It is thus subjected to a cation
as well as an anion exchange.
[0095] The cation exchange (e.g. on a "Duolite
R"-C20 resin) is performed at a temperature of 25-35°C in order to avoid hydrolysis
of the oligosaccharides.
[0096] During the anion exchange (e.g. on a "Duolite
R" A-378 resin) the coloured compounds of the juice are also removed as to render said
juice a colourless oligosaccharide solution. The dry matter content after the ion
exchange is 8-14% by weight.
7. Treatment with active carbon
[0097] It may necessary to treat the ion-exchanged juice with active carbon in order to
remove possible residues of coloured compounds, undesired taste or odoriferous compounds.
8. Evaporation
[0098] Before the actual evaporation it is advantageous to employ hyperfiltration (reverse
osmosis) in order to remove part of the water so that the dry matter content is up
to approx. 25% by weight. By this step a more gentle treatment is obtained.
[0099] The evaporation is performed in a multi-step evaporator such as a falling film evaporator.
The juice is evaporated to a syrup of a dry matter content of between 75-85% by weight.
[0100] Thereafter the syrup is evaporated in a vertical vacuum dryer or a thin film evaporator
to a dry matter content of 90-96% by weight, preferably 91-93% by weight.
Example 2: Vacuum flash drying
[0101] A syrup having a dry matter content of 91-93% by weight obtained according to the
method of Example 1 and being of a temperature of 80-100°C is transferred to a vacuum
chamber provided with a conveyor belt.
[0102] By adjusting the dry matter content and the temperature of the feeding material as
well as the vacuum in the chamber the obtained product has a temperature of 30-40°C
after evaporation of water and is solid. The heat of evaporation is derived from the
enthalpy of the feeding material, i.e. it is
not necessary to add heat during the drying process.
[0103] At an absolute pressure of 23.8 or 42.2 mmHg the product leaving the vacuum chamber
has a temperature of approx. 30°C or approx. 40°C respectively.
[0104] The process can be described as a flash-like evaporation in vacuum, the feed being
a syrup and the final product a dry powder.
[0105] The above process differs from conventional flash evaporation by being performed
in vacuum, thus rendering it unnecessary to overheat the feeding material, and by
the feeding material being a solution and not a wet, particulate matter.
[0106] An interesting property of this drying method is the fact that the product is cooled
to a desired final temperature of typically 30-40°C during the drying/water evaporation.
Example 3
[0108] The general procedure described in Example 2 is carried out in a vacuum chamber provided
with a self-cleaning twin screw. In this way a final dry powder product similar to
the product obtained in Example 2 is obtained.
Example 4
[0109] A syrup having a dry matter content of 91% by weight and a temperature of 95°C obtained
according to Example 1 is fed into a vacuum chamber provided with a conveyor belt.
The absolute pressure in the vacuum chamber is 25 mm Hg. The dry powder leaving the
chamber has a dry matter content of 96% by weight and a temperature of 31°C.
Example 5
[0110] A syrup having a dry matter content of 93% by weight and a temperature of 95°C obtained
according to Example 1 is fed into a vacuum chamber provided with a conveyor belt.
The absolute pressure in the vacuum chamber is 39 mm Hg. The dry powder leaving the
chamber has a dry matter content of 98% by weight and a temperature of 39°C.
Example 6
[0111] A syrup having a dry matter content of 92% by weight and a temperature of 85°C obtained
according to Example 1 is fed into a vacuum chamber provided with a conveyor belt.
The absolute pressure in the vacuum chamber is 30 mm Hg. The dry powder leaving the
chamber has a dry matter content of 95.8% by weight and a temperature of 35°C.
Example 7
[0112] A syrup having a dry matter content of 91% by weight and a temperature of 99°C obtained
according to Example 1 is fed into a vacuum chamber provided with a self-cleaning
twin screw. The absolute pressure in the vacuum chamber is 30 mm Hg. The dry powder
leaving the chamber has a dry matter content of 96.5% by weight and a temperature
of 35°C.
Example 8: Drying of glucose syrup
[0113] The starting material is a commercially available glucose syrup with a dry matter
content of 80% by weight. The syrup is pre-evaporated by means of batch evaporation
for 2 h in a vertical vacuum dryer to a dry matter content of 92.5% by weight. During
the evaporation the syrup has a temperature of 85°C and the vacuum is 80% (absolute
pressure about 150 mm Hg). After the batch evaporation the temperature is elevated
to 95°C and the syrup is extruded into a vacuum flash dryer, as shown in Fig. 1. The
gap of the extruder has a width of 0.5 mm. In the vacuum flash dryer the absolute
pressure is 24 mm Hg.
[0114] After leaving the extruder the syrup foams up momentarily to a thickness of 5-6 cm.
After a sojourn time of 1 1/2 min the dried syrup leaves the vacuum flash dryer at
a temperature of 35°C in form of a rough granulate with a dry matter content of 97.5%
by weight.
[0115] Fig. 3 is a diagram illustrating the possibilities of altering the dry matter content
and the temperature of the input material and still end up with the same final product.
In Fig. 3 the abscissa represents % by weight of the dry matter content in the input
material and the ordinate represents the dry matter content in % by weight of the
material leaving the assembly.
Example 9
[0116] A mixture comprising 80% by weight of glucose syrup having a dry matter content of
80% by weight and 20% by weight of concentrated apple juice having a dry matter content
of 67% by weight is evaporated in a vertical vacuum dryer to a dry matter content
of 92.5% by weight. After the evaporation the temperature is adjusted to 97°C and
the material is extruded through nozzles having a diameter of 1 mm into a vacuum flash
dryer with free fall as shown in Fig. 4. In the vacuum flash dryer the absolute pressure
is 10 mm Hg. The dry powder leaving the vacuum flash dryer has a dry matter content
of 96.3% by weight and a temperature of about 34°C.
Example 10
[0117] In a vertical vacuum dryer cane sugar molasses having a dry matter content of 80%
by weight is evaporated to a dry matter content of 93% by weight, whereupon the temperature
of the material is adjusted to 96°C. The molasses is extruded into the vacuum flash
dryer of Example 9, the absolute pressure in said dryer being 14 mm Hg. A dry amorphous
powder leaving the vacuum flash dryer has a dry matter content of 96.3% by weight
and a temperature of about 32°C.
[0118] This example demonstrates the possibility of obtaining an amorphous powder instead
of the crystalline form obtained by conventional methods.